How do Vodafone and other networks distribute airtime to retailers like carfone warehouse?

Answer 1

Not sure what you mean .

The Mobile Network operates via a series of Base Stations ('mobile masts')... your phone connects to the nearest base station automatically 'on demand' .. there is no 'distribution' going on .. these days most base stations support all the networks .. (I would guess that which-ever company maintains an individual base station will charge 'rent' to the other companies)

If there are too many mobiles operating near one base station then it simply ignores any it can't handle .. (you either get 'no signal' or 'busy')